Added coroutines to integration tests

Changed all prepared_statement functions to not use auto return type
declaration
Fixed wrong return value in prepared_statement::execute

examples/metadata.cpp

@@ -31,7 +31,7 @@ void main_impl(int argc, char** argv)
 	// TCP and MySQL level connect
 	boost::asio::io_context ctx;
 	boost::mysql::tcp_connection conn (ctx);
-	conn.next_level().connect(ep);
+	conn.next_layer().connect(ep);
 	conn.handshake(params);
 
 	// Issue the query